The present invention relates generally to the industrial vehicle field and more particularly, to a forklift truck providing both extensive positive (upward above ground level) and negative (downward below ground level) lift capabilities such as required of, for example, “marina” type lifts.
Certain applications of forklift trucks require an upright construction that is capable of providing both positive and negative lift from a ground or support level position. For example, such a forklift truck is particularly useful for handling boats in and around marinas. The market for such a forklift truck has significantly increased in recent years with ever more and more people owning and operating pleasure boats.
Forklift trucks may be used in warehouses or other structures where loads must be raised above ground level for storage or lowered below ground level for various reasons. In the marina setting, forklift truck may be utilized to both lower boats into and raise boats out of the water from an elevated dock or the like. Similarly, such forklift trucks may be utilized to raise boats for positioning well above the ground in an overhead storage rack.
Heretofore, forklift truck designs have been developed for this purpose. One such representative design is disclosed in U.S. Pat. No. 3,841,442 to Erickson et al. The forklift truck disclosed in the Erickson et al patent includes outer, intermediate and inner, telescoping mast sections with a load carriage elevatable on the inner mast section. The forklift truck also includes a pair of actuator cylinders and cooperating chains. These cylinders and chains are connected to the mast sections so that one cylinder and chain set is adapted to elevate the load carriage and the inner mast section above ground level. The other cylinder and chain set is adapted to lower below ground level the load carriage and inner and intermediate mast sections together as a unit in the outer mast section.
Another representative design is disclosed in U.S. Pat. No. 5,326,217 to Simpson et al. The forklift truck disclosed in Simpson et al. includes first, second and third telescoping mast sections. Each mast section is formed from pairs of laterally spaced and interconnected I-beam rails. A carriage assembly is mounted for movement along a path on the third mast section. Rollers are provided for mounting the mast sections and carriage assembly together for relative movement. A drive assembly connected to the upright assembly and carriage assembly serves to move the carriage assembly at a first, relatively slow speed over a first portion of the movement path and at a second relatively fast speed over a second portion of the movement path. More particularly, the drive assembly includes twinned actuating cylinders for operatively connecting the first and second mast sections. A first dead chain operatively connects the first and third mast sections. A second dead chain operatively connects the second mast section and the carriage assembly. A guide rod is connected to one end of the second dead chain and a guide sleeve is mounted to the second mast section. The guide rod is received in free sliding engagement in the guide sleeve so as to prevent relative movement between the carriage assembly and the third mast section at all times when the carriage assembly is in a negative lift configuration, that is, below ground level. A stop is mounted to the distal end of the guide rod. When the carriage assembly is in a positive lift configuration, that is, above ground level, the stop abuts against and engages the guide sleeve. This engagement causes relative movement of the carriage assembly along the third mast section when in any positive lift configuration.
A forklift mast assembly typically consists of two or more sections of track nestably engaged in a vertical orientation for elevating a set of forks above ground level. Such elevation is typically achieved through the action of systems including cylinders, pulleys and/or chains. Through varying configurations of cylinders, pulleys and/or chains, prior art forklift mast assemblies have been able to achieve varying ratios of relative movement of the track sections and forks.
Forklift trucks must be capable of entering and exiting doorways on structures in which loads are to be stored. The mast assembly must be sufficiently low to permit entrance and egress through the doorways on the structures. Certain configurations with improved height characteristics have been able to achieve negative-lift or the lowering of the forks below the level of the wheels on the truck. Other assemblies have been able to achieve free-lift or the free movement of the forks along the track section without increasing the overall height of the lowered mast assembly.
These prior art assemblies and configurations have been plagued with drawbacks such as an overall lowered height that is too high for entry into certain structures. Certain configurations have also been unable to achieve the combination of free-lift and negative-lift. Other configurations, while able to achieve a lower overall height have done so by adding additional track sections resulting in an overall heavier mast assembly.
Therefore, there is a need for a forklift mast assembly that provides a lower overall lowered height with fewer track sections while providing negative-lift and free-lift. The present invention fulfills these needs and provides other related advantages.